Process for minimizing pellicle fouling



May 30, 1967 w THOMPSON PROCESS FOR MINIMIZING PELLICLE FOULING- Filed Feb. 10, 1964 United States Patent 3,322,564 PROCESS FOR MINIMIZING PELLICLE FOULING Gordon W. Thompson, Clinton, Iowa, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Feb. 10, 1964, Ser. No. 343,850 6 Claims. (Cl. 117-102) This invention relates to an improved method of coating pellicles and sheets. More particularly it relates to an improvement in reducing the amount of waste produced in the continuous process of coating fibrous and nonfibrous pellicles and sheets.

In the process of applying a coating to a pellicular material, the uncoated sheet is generally unwound, passed over and around various rolls to properly tension the web, passed through a station where the coating is applied in any one of a number of ways such as from a kiss roll or by dipping foil-owed by doctoring, passed through a heated section where the coating and base sheet are dried, may be optionally passed through a humidifier in the case of pellicles having hydrophilic layers such as regenerated cellulose or through a heat-treating or conditioning stage, again passed over various rolls to properly tension the web and is rewound in roll form. Coating towers are constructed in either a horizontal or vertical arrangement with the web passing around or over one or more transfer rolls between the dryer and subsequent conditioner. Bowed rolls, termed spreader rolls, are placed strategically at various locations in the apparatus to assure that the web is firmly spread out in the transverse direction, that is, to prevent the formation of wrinkles and folds. One such strategic location for a spreader roll is immediately before the pellicle reaches a transfer roll, at which location it prevents wrinkles and folds from occurring and being pressed into the freshly coated and dried pellicle.

When such coating apparatus is operated in continuous fashion a change from one mill roll of base sheet to the next is made without shutting down the equipment. When the coating is applied by doctor rolls, blades, knives, or the like, that is, when the coating is applied at a constricted area through which the pellicle travels, the size of the opening presented to the web is limited, and is determined in part by such factors as the thickness of the web, the amount of coating to be applied to it, and the concentration of the coating ingredients in the coating bath. In a continuous coating operation when a flying mend is made in changing from one roll of base sheet to the next, the tail of one roll is secured to the lead end of the next roll by gluing, taping, or the like, and in such manner the new roll is threaded through the coating machine by the previous one. In this operation, at least two layers of the pellicle, and frequently more than two as a result of temporary folds and wrinkles in either layer, are presented to and must pass simultaneously through the constriction at which the coating is applied. If the constriction is not made larger at this time, invariably either the mend tears apart or the pellicle itself breaks. Consequently, a coating apparatus which is intended to operate in continuous manner always contains provision for enlarging the said constriction when the mend is passed through the apparatus. It is clear that when the said constriction is enlarged the operation of the constriction to limit the amount of coating applied to the pellicle is ineffective. That part of the pellicle at and in the vicinity of the mend thus carries into the dryer an amount of coating bath in excess of the normal amount and when the coating apparatus is operating in continuous fashion, that is, running at normal operating speeds, the drying capacity of the machine is inadequate to properly and completely dry the amount of coating passing therethrough. lf the coating is applied by a kiss roll, again the mend picks up more than the standard amount of coating, carries it into the dryer, and is similarly not completely dried. The incompletely dried coating, which is generally rather sticky, will tend to adhere and transfer to whatever it first encounters, in this case the spreader roll immediately before the transfer roll, thus fouling the said spreader roll with deposits of partially dried coating. After the mend, as additional pellicle carrying a normal thickness of completely dried coating passes over the fouled spreader roll, the partially dried coating on the spreader roll is gradually pulled off onto the pellicle in bits and pieces over a period of time, thus contaminating the finished product, degrading its appearance, and causing consider-able loss and waste due to substandard quality. To this time, no completely satisfactory mode of operating a coating apparatus which avoids the loss of considerable product has been known. 7

It is therefore an object of this invention to reduce the amount of substandard coated product manufactured in a continuous coating operation. Another object is to provide a method of cleaning spreader rolls in coating apparatus which does not entail shutting down the coating operation but allows continuous operation. A further object is to provide a practical method for maintaining continuous, relatively waste-free, 'high speed operation of coating apparatus in order to maintain a high level of productivity on this complex and expensive apparatus. These and other objects will become more apparent hereinafter. The objects of this invention are accomplished in a continuous process for coating a pellicle wherein when flying mends are employed to thread successive rolls of pellicle through coating apparatus and newly coated pellicle contacts a spreader roll upon passing from the drying chamber, more than the usual amount of coating is carried through said drying chamber by said pellicle in the vicinity of the mend and said spreader roll becomes fouled with partially dried coating; the improved method for minimizing fouling which consists in driving said spreader roll at a speed of about 10% to 100% faster than the speed of said pellicle for a period of time sufficient to clean from the spreader roll substantially all of the coating adhering thereto.

The exact time at which the spreader roll is speeded up is not critical to the operation of the invention and may be just before, while, or just after the mend passes the spreader roll. However, it is preferred to speed up thespreader roll just before the mend passes over it.

The speed at which the spreader roll is overdriven may be varied widely, but it is preferred to drive it at speeds of about 30 to beyond web speed during the cleaning step.

The figure is a schematic side elevation of one type of coating apparatus in which the present invention may be employed. In the apparatus shown the uncoated material 1 is unwound from the roll 2, and is passed over various tensioning rolls 3 as it enters coating compartment 4 through slit 35. The web then contacts spreader roll 5 and passes below roll 6 submerged beneath the surface of coating lacquer 7 contained in dip pan 8. The web then passes upwardly between doctor rolls 9 into drying compartment 10- through slit 36. Hot air heated by heater 11 is circulated into compartment 10 by means of blower '12 through duct 13. The solvent or water-laden air is exhausted from compartment 10* through duct 14. The web then passes into cooling compartment 15 through slit 37 and passes over spreader roll 16 and transfer roll 17. Cooled air may be forced into this compartment if desired through duct 18 and exhausted through duct 19. The web then passes downwardly through slit 38 into humidifying or conditioning section 20. Humidified air is prepared in humidifying compartment 21 and is forced through duct 22 by blower 23 into compartment 20. The exhaust from compartment 20 exits through duct 40. The web passes downwardly out of compartment '20 through slit 39, passes in contact with spreader roll 24, various tensioning rolls 25, spreader roll 26, and is collected as a roll of coated product 27. Roll 28 serves to press the sheet flat as it is collected in roll form.

Numeral 29 indicates generally a two-position unwind rotatable about shaft 30. The two-position unwind 29 holds not only uncoated roll 2 which is feeding into the apparatus, but also a full roll of uncoated sheet 31 in a ready position. When roll 2 is almost exhausted, two-position unwind 29 will be rotated around shaft 30 so that the lead end of roll 31 may be attached to the tail of roll 2 in order to carry it through the apparatus. Numeral 32 indicates generally a two-position windup rotatable about shaft 33. When a full roll of coated product 27 has been collected, windup 32 will be rotated about shaft 33 to bring mandrel 34 into position to collect the next roll of coated product.

As can readily be seen, spreader roll 16 is of necessity disposed in a location inconvenient and somewhat inaccessible to the operator. Cleaning of the roll with long handled brushes or tools is not only difiicult but also dangerous for the operator. Further, such manual methods frequently result in breaking of the pellicle, thus shutting down the operation.

The spreader roll 16 may be any known type of spreader roll which is capable of being driven at speeds in excess of the web speed. Examples of such rolls which can be used in this invention are described by Warner in United States Patents 2,712,681 and 2,712,682, and by Weiss- Oeschger in United States Patent 1,668,994.

As stated hereinabove the spreader roll is driven beyond the web speed for a period of time sufficient to clean from the P011 the adhering coating fouling it. The actual amount of time will depend on such factors as the speed of the coating operation, the nature of the coating, and the amount by which the spreader roll is driven beyond the web speed. The relation between the amount of excess roll speed and time for cleaning is inverse, the greater the amount by which the spreader roll is overdriven, the shorter the time required for cleaning. In general, it is found that when the spreader roll is driven about 30 to 80% beyond film speed, cleaning of the fouled roll is accomplished in a matter of to 15 seconds, whereas when the spreader roll is allowed to idle or is driven only at film speed according to previous practice, as long as five minutes or more may be required before cleaning of the fouled roll is completed. It is apparent that at the speed of typical coating operations, as much as two thousand feet or more of pellicle may be saved per mend. As a result of the practical limitations on the size of rolls of the pellicle which can be conveniently handled, the saving may be as much as of the pellicle coated. This is not an inconsiderable amount when it is realized that the finished product which carries the contaminating spots of coating must be either destroyed, recovered, or relegated to being sold as substandard quality.

The process of this invention can be employed in coating a wide variety of hydrophilic and hydrophobic base sheets, including regenerated cellulose, cellulose esters such as cellulose acetate, cellulose ethers such as methyl cellulose, polyolefin films such as polyethylene and polypropylene, polyester-s such as polyethylene terephthalate, polyamides such as the nylons, cellulosic papers, and woven and nonwoven fabrics.

This invention is useful with many different types of coatings. It may be used with coatings applied as aqueous solutions or dispersions, such as polymers of vinylidene chloride and copolymers thereof, polyvinyl alcohol, and hydrolyzed ethylene-vinyl acetate copolymers. It may also be advantageously used when the coating is applied from solvent solution, as in the case of nitrocellulose coatings,

polymers or copolymers of vinylidene chloride, vinyl chloride, vinyl acetate, acrylonitrile, acrylate esters, methacrylate esters, and the like.

Although the invention has been described as being used when a flying mend is made, it should be evident that the method is equally useful at other times when a spreader roll becomes fouled, such as when the coating machine is started up after being idle for any reason, since excess coating will be carried through the machine when a new roll of base sheet is initially threaded therethrough.

The invention has been described with specific reference to one location Where a spreader roll may be overdriven to advantage. It is apparent that at other locations where spreader rolls are placed, overdriving of the rolls for the purpose of cleaning them may be equally advantageous.

The following examples are illustrative embodiments of this invention but are not intended as limitations thereon. In the examples, all parts indicated are parts by weight.

Example I Rolls of polypropylene base sheet were coated in a kiss roll coating apparatus with an aqueous dispersion of a copolymer consisting of 91 parts vinylidene chloride, 8 parts methyl acrylate, and 1 part itaconic acid, in continuous fashion, while making flying mends. During flying mends, the spreader roll at the exit of the dryer was driven beyond film speed by 70%. The roll was invariably thoroughly cleaned within 5 to 15 seconds. During control tests at the same film speed when the spreader roll was driven only at film speed, cleaning of the spreader roll aways required at least 30 seconds and frequently as long as 5 minutes, as gyidenced by foreign spots of excess coating stuck to the Example II Rolls of regenerated cellulose base sheet were coated in a doctor roll apparatus with a heat-scalable nitrocellulose coating composition in a continuous manner while making flying mends. During mends the spreader roll at the exit of the dryer was driven beyond film speed by 35%. The roll was invariably thoroughly cleaned within 5 to 10 seconds. During control tests at the same film speed with the spreader roll rotating only at film speed the cleaning of the spreader roll required at least 20 seconds and sometimes as long as 4 minutes.

Example III Rolls of a low basis weight (7.5 lbs./17 in. x 22 in. ream) manifold paper were coated in an apparatus where coating weight was controlled by a doctor knife with a solution of a coplymer consisting of 91 parts vinylidene chloride and 9 parts acrylonitrile in a continuous operation while making flying mends. At each mend the spreader roll at the exit of the dryer was driven beyond web speed by 50%. The roll was invariably cleaned within 5 to 10 seconds. During control tests at the same web speed with the spreader roll rotating only at web speed, cleaning of the roll required between 25 seconds and 5 minutes.

The method of cleaning fouled spreader rolls which is provided by the present invention offers advantages over the previously employed operating methods. These advantages includes simplicity and safety of the method, the expeditious manner in which it is carried out, and the significant reduction in the amount of waste produced in the coating operation.

It is clear that various changes, modifications and embodiments not explicitly illustrated herein may be made in this invention without departing from the spirit or scope thereof which is accordingly intended to be limited only by the scope of the appended claims.

What is claimed is:

1. In a continuous process for coating a pellicle, wherein when flying mends are employed to thread successive rolls of pellicle through coating apparatus and newly coated pellicle contacts a spreader roll upon passing from the drying chamber of said apparatus, more than the usual amount of coating is carried through said drying chamber by said pellicle in the vicinity of said mend and said spreader rol-l becomes fouled with partially dried coating; the improved method for minimizing fouling which consists in driving said spreader roll at a speed of about to 100% faster than the speed of said pellicle for the period of time starting between just .before and just after the mend passes over said spreader roll and continuing until such time that substantially all of the coating adhering to said spreader roll has been removed therefrom.

2. The improved method of claim 1 wherein said spreader roll is driven at a speed of about 30% to 80% faster than the speed of said pellicle.

3. In a continuous process for coating a pellicle, wherein when flying mends are employed to thread successive rolls of pellicle through coating apparatus and newly coated pellicle contacts a spreader roll upon passing from the drying chamber of said apparatus, more than the usual amount of coating is carried through said drying chamber by said pellicle in the vicinity of said mend and said spreader roll becomes fouled with partially dried coating; the improved method for minimizing fouling which consists in driving said spreader roll at a speed of about 10% to 100% faster than the speed of said pellicle for the period from just before said mend passes over said spreader roll until such time that substantially all of the coating adhering to said spreader roll has been removed therefrom.

4. The improved method of claim 3 wherein said spreader roll is driven at a speed of about to faster than the speed of said pellicle.

5. The process of claim 3 wherein said pellicle is polypropylene .base sheet.

6. The process of claim 3 wherein said pellicle is cellulose base sheet.

No references cited.

ALFRED L. LEAVJTT, Primary Examiner. A. H. ROSENSTEIN, Assistant Examiner. 

1. IN A CONTINUOUS PROCESS FOR COATING A PELLICLE, WHEREIN WHEN FLYING MENDS ARE EMPLOYED TO THREAD SUCCESSIVE ROLLS OF PELLICLE THROUGH COATING APPARATUS AND NEWLY COATED PELLICLE CONTACTS A SPREADER ROLL UPON PASSING FROM THE DRYING CHAMBER OF SAID JAPPARATUS, MORE THAN THE USUAL AMOUNT OF COATING IS CARRIED THROUGH SAID DRYING CHAMBER BY SAID PELLICLE IN THE VICNITY OF SAID MEND AND SAID SPREADER ROLL BECOMES FOULDED WITH PARTIALLY DRIED COATING; THE IMPROVED METHOD FOR MINIMIZING FOULING WHICH CONSISTS IN DRIVING SAID SPREADER ROLL AT A SPEED FOR ABOUT 10% TO 100% FASTER THAN THE SPEED OF SAID KPELLICLE FOR THE PERIOD OF TIME STARTING BETWEEN JUST BEFORE AND JUST AFTER THE MEND PASSES OVER SAID SPREADER ROLL AND CONTINUING UNTIL SUCH TIME THAT SUBSTANTIALLY ALL OF THE COATING ADHERING TO SAID SPREADER ROLL HAS BEEN REMOVED THEREFROM. 